High-pressure electron tunneling spectroscopy reveals the presence of a superconducting gap in H₃S and D₃S.
Superconductors are materials that can conduct electrical current without any resistance, making them crucial for applications such as energy transmission, magnetic levitation, energy storage, and quantum computing.
Traditionally, superconductivity has only been observed at extremely low temperatures, which has limited its practical applications. A major breakthrough came with the discovery of superconductivity in hydrogen-rich compounds, notably hydrogen sulfide (H₃S) and lanthanum decahydride (LaH₁₀). H₃S exhibits superconductivity at 203 Kelvin (-70°C), while LaH₁₀ achieves superconductivity at 250 Kelvin (-23°C). These milestones brought researchers significantly closer to the long-sought goal of room-temperature superconductivity.
Because their critical temperatures are well above the boiling point of liquid nitrogen (77 K), these materials are classified as high-temperature superconductors.
A fundamental aspect of superconductivity is the superconducting gap, a key property that reflects how electrons pair to create the superconducting state. Identifying this gap is essential to distinguishing the superconducting phase from ordinary metallic behavior.
However, measuring the superconducting gap in hydrogen-rich materials like H₃S has proven extremely challenging. These compounds must be synthesized under ultra-high pressures, over one million times atmospheric pressure, inside diamond anvil cells. Under such extreme conditions, traditional measurement techniques like scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) are not feasible.
Tunneling technique provides direct insight into the superconducting state of hydrogen-rich compounds
To overcome this barrier, researchers at the Max Planck Institute in Mainz developed a planar electron tunneling spectroscopy capable of operating under such extreme conditions. This achievement has enabled them to probe the superconducting gap in H3S for the first time, offering direct insight into the superconducting state of hydrogen-rich compounds.
Using this technique, the researchers discovered that H3S exhibits a fully open superconducting gap with a value of approximately 60 millielectronvolt (meV), while its deuterium analogue, D3S, shows a gap of about 44 meV. Deuterium is a hydrogen isotope and has one more neutron. The fact that the gap in D3S is smaller than in H3S confirms that the interaction of electrons with phonons – quantized vibrations of the atomic lattice of a material – causes the superconducting mechanism of H3S, supporting long-standing theoretical predictions.
For the Mainz researchers, this breakthrough is not just a technical achievement – it also lays the foundation for fully unraveling the origin of high-temperature superconductivity in hydrogen-rich materials. “We hope that by extending this tunneling technique to other hydride superconductors, the key factors that enable superconductivity at even higher temperatures can be pinpointed. This should ultimately enable the development of new materials that can operate under more practical conditions,” states Dr. Feng Du, first author of the now published study.
Dr. Mikhail Eremets, a pioneer in the field of high-pressure superconductivity who passed away in November 2024, described the study as “the most important work in the field of hydride superconductivity since the discovery of superconductivity in H3S in 2015.” Vasily Minkov, project leader of High-Pressure Chemistry and Physics at the Max Planck Institute for Chemistry commented: “Mikhail´s vision of superconductors operating at room temperature and moderate pressures comes a step closer to reality through this work.”
About Superconductivity
Superconductivity is a remarkable property of materials to conduct electrical current without resistance. Discovered in pure mercury by Heike Kamerlingh Onnes in 1911, this phenomenon was long believed to exist at extremely low temperatures, close to absolute zero (–273 °C). That paradigm shifted in the late 1980s when Georg Bednorz and Karl Alexander Müller discovered a new family of cupper-oxide (cuprate) superconductors that exhibited high-temperature superconductivity under atmospheric pressure.
A wave of global research followed, eventually reaching a critical temperature (Tc), the temperature at which a material loses its resistance, of approximately 133 K at ambient pressure and 164 K under high pressure. However, no superconductor with a higher Tc had been discovered – until the advent of hydrogen-rich compounds.
The discovery of superconductivity in H3S at megabar pressures, with a Tc = 203 K by the research group led by Dr. Mikhail Eremets, thus marked a revolutionary advance towards achieving superconductivity near room temperature. This breakthrough was soon followed by discoveries of even higher Tc values in hydrogen-rich metal hydrides, such as YH9 (Tc ≈ 244 K) und LaH10 (Tc ≈ 250 K). Theoretical models now predict superconductivity above room temperature in several hydrogen-dominated systems under extreme pressures.
About Cooper pairs and the Superconducting gap
In ordinary metals, electrons with energy states near the Fermi level can flow freely. The Fermi level corresponds the highest energy level that electrons can occupy in a solid at absolute zero. However, when a material becomes superconducting, electrons form so-called Cooper pairs, entering a collective quantum state. As a highly correlated state, the Cooper pair of electrons moves like a single entity without scattering with phonons or impurities in the crystal structure of the material and therefore has no resistance.
This pairing is characterized by an energy gap near the Fermi level – the superconducting gap – which is the minimum energy needed to break a Cooper pair of electrons. The existence of the gap protects the superconducting state from disturbances like scattering.
The superconducting gap is the defining feature of a superconductor’s quantum state. Its value and symmetry offer critical insights into how electrons interact and pair, serving as a fingerprint of the superconducting mechanism.
Reference: “Superconducting gap of H3S measured by tunnelling spectroscopy” by Feng Du, Alexander P. Drozdov, Vasily S. Minkov, Fedor F. Balakirev, Panpan Kong, G. Alexander Smith, Jiafeng Yan, Bin Shen, Philipp Gegenwart and Mikhail I. Eremets, 23 April 2025, Nature.
DOI: 10.1038/s41586-025-08895-2
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3 Comments
The superconducting gap is the defining feature of a superconductor’s quantum state. Its value and symmetry offer critical insights into how electrons interact and pair, serving as a fingerprint of the superconducting mechanism.
VERY GOOD.
Symmetry provides us with key fingerprints for understanding natural laws. According to the topological vortex theory (TVT), so-called “quantum materials” are merely one manifestation of the diverse topological materials found in nature. There is no need to mystify our understanding of quantum phenomena by invoking analogies like Schrödinger’s cat (a cat being both dead and alive simultaneously).
Topological Vortex Theory (TVT), by redefining spacetime as a dynamic network of topological vortices, reveals the extraordinary complexity of vortex superposition states, fractal entanglement, and locking mechanisms in topological materials. Key contributions include: (1) the multidimensional nature of topological vortex superposition states; (2) interaction patterns surpassing quantum superposition; (3) new physical dimensions enabled by fractal structures and dynamic locking mechanisms.
Superconductivity